The present invention relates to an economical process for arc welding thick plates of pure copper or copper alloys from one side only in an atmosphere of an inert gas with the use of a backing support.
In recent years, pure copper or copper alloy materials have been used in the production of various apparatus and articles including industrial furnaces and electronic components and parts because of their excellent thermal conductivity, electrical conductivity, strength and ductility. In the production of these furnace and parts, a gas shielded arc welding process has been often utilized. Welding of these copper materials entails two problems to be considered which are not encountered in the welding of steel. First, copper or its alloy to be welded has good fluidity and tends to fall off the welding part or area. The second problem is that because the copper materials have good thermal conductivity, they require a greater heat input than the net heat required for fusing a filler metal, and it is nonetheless difficult to obtain a satisfactory joint with a sufficient penetration.
Heretofore, in order to overcome the first problem, a backing support consisting of a copper or water-cooled copper plate has been placed on the underside of the welding area of the copper plates and welding of the copper plates has been carried out from the opposite side of the backing support. However, the use of these backing supports entails the following disadvantages.
(1) It is difficult to shield the area to be welded from the side where the backing support is placed and the moisture or oxygen present in air dissolves into the metal to be welded, thus causing blow holes or cracks in the metal and at the same time embrittling the metal.
(2) Because of its relatively low melting point, the backing support may be melt-bonded to the joint, and removal of the support thus bonded is troublesome.
On the other hand, it is known to use a heat-resistant inert material such as an asbestos sheet and a carbon plate as a backing support. Among these, a backing support made of carbon has been found to be particularly good. This is not simply because the backing support made of carbon is heat resistant and inert, but rather because it creates a reducing atmosphere in the vicinity of an underside or back bead due to the heat of welding, so that it is possible to produce a sound weld by preventing the back bead from being oxidized without further shielding with an inert gas.
Also, the carbon backing support has an advantage in that it makes possible the formation of a flat back bead by welding from only one side of the copper plates to be welded without post-treatment after welding under suitable welding conditions because the support is non-reactive with molten metals. Further, the carbon support is a good heat insulator.
The second problem of heat loss is more troublesome. Welding of copper materials requires a heat input which is about 7 times as much as that required in the welding of steel in order to obtain the same welding performance because copper materials have a high thermal conductivity. If this large quantity of heat is not supplied, satisfactory welding results cannot be obtained. This heat loss problem obviously becomes conspicuous particularly when a pair of the copper plates to be welded (hereinafter sometimes referred to collectively as "parent plate") is increased in thickness because the heat capacity of the welding area or part is correspondingly increased.
One approach to solve the heat loss problem which has been heretofore proposed is to increase the heat input by increasing the welding current. However, this method is disadvantageous in that it results in a high proportion of defective welds, and, in particular, when the above mentioned carbon backing support is used, the support is subjected to an excessive heat input and perforating action of the arc, whereby blow holes are formed in the weld due to decomposition of the support.
A second approach to solve the heat loss problem is to pre-heat the parent plate to a temperature of 100.degree. to 600.degree. C. This method is certainly effective for reducing the requirement of a high heat input to be supplied by an arc. However, this method requires a pre-heating process and apparatus therefor which renders it disadvantageous from the viewpoint of economy.
A third approach to a solution of the heat loss problem is to form a groove, for examle, in a V or U shape, in the welding part to reduce the heat capacity of the part. This method requires a machining process prior to welding and, therefore, is uneconomical.
Heretofore, a combination of the pre-heating and the groove preparation has been generally used for welding copper plates having a thickness of 5 mm or more. However, such a process has the various disadvantages as stated above.